Electrical connectors are used in electronic equipment and devices to communicate electrical signals from one printed circuit board to another. As operating speeds of the electronics of such electronic equipment and devices have increased, the communication of the electrical signals in a noise-free fashion has become more important and more difficult to achieve. If, for example, an electrical signal is transmitted down a conductor and if there are discontinuities in the characteristic impedance of the conductor, or if the conductor is not properly terminated, then electrical reflections may be generated. These reflections are undesirable and may obscure the desired signal that was to be conducted down the conductor. If, for example, two conductors extend parallel and close to one another for a long distance, a signal propagating down one of the conductors may induce a signal into the other conductor. Again, the induced signal is undesirable and may obscure a desired signal that was to be conducted down the other conductor. If, for example, an adequately long segment of a conductor is left unshielded and if a high frequency signal is present on the segment, then the segment may act as an antenna and radiate electromagnetic radiation or receive electromagnetic radiation. This is undesirable as well. As the operating speeds of the electronics within the electronic equipment and devices have increased over time, the need to minimize reflections, cross-talk and the radiation of electromagnetic energy in the conductors within electrical connectors has become more important.
FIG. 1 (Prior Art) is a simplified perspective view of a piece of electronic equipment 1 such as a router or computer. Equipment 1 includes a first printed circuit board 2 extending in a first plane and a second printed circuit board 3 extending in a second plane perpendicular to the first printed circuit board. The first printed circuit board is often referred to as a motherboard or a backplane. The second printed circuit board is often referred to as a daughterboard or line card or expansion board. Although not illustrated in FIG. 1, there are typically many daughterboards within the piece of electronic equipment.
Electrical signals are communicated between first printed circuit board 2 and second printed circuit board 3 across a right angle connector assembly. The connector assembly includes a first connector 4 disposed on the motherboard and a second connector 5 disposed on the daughterboard. The first connector 4 is often referred to as the motherboard connector and the second connector 5 is often referred to as the daughterboard connector. The assembly is called a right angle connector because the two printed circuit boards are disposed at right angles with respect to one another.
FIG. 2 (Prior Art) is an expanded perspective view of motherboard 2, motherboard connector 4, daughterboard 3, and daughterboard connector 5. To couple the daughterboard to the motherboard, the daughterboard is moved with respect to the motherboard in the direction of arrow 6 such that female daughterboard connector 5 mates with male motherboard connector 4. Individual signal conductors within daughterboard connector 5 are thereby coupled to corresponding individual signal conductors within motherboard connector 4.
FIG. 3 (Prior Art) is a cross-sectional diagram showing how motherboard connector 4 is mechanically and electrically coupled to motherboard printed circuit board 2. Daughterboard connector 5 is coupled to daughterboard 3 in similar fashion. Motherboard connector 4 is a male connector that includes an insulative housing 7 and a plurality of metal pins 8 and 9. Each pin has a first end for mating with female daughterboard connector 5 and a second press-file contact tail end. Each press-fit contact tail extends into a corresponding through hole in the printed circuit board. There are two press-fit contact tails 10 and 11 illustrated in FIG. 3. Each contact tail has a hollow eye which allows the contact tail to be compressed by the sidewalls of the through hole as the contact tail is forced into the through hole when connector 4 is fixed to motherboard 2. The contact tail presses back out against the sidewalls of the through hole and thereby holds the contact tail and pin in place. All the contact tails of the all the pins in turn hold the connector 4 in place on the printed circuit board.
FIG. 4 (Prior Art) is an end view of male motherboard connector 4. Insulative housing 7 includes a first sidewall portion 12 and a second sidewall portion 13. The ends of pairs of numerous signal pins are seen extending upward toward the viewer from the plane of the page. Pins 8 and 9 are one such pair. The signal pins are disposed in pairs because differential electrical signals are conducted over the signal conductors. The electric signal being communicated is a differential signal between a signal on the first signal pin of the pair and the second signal pin of the pair.
In addition to pairs of signal pins, a plurality of vertically oriented ground strips 15 is illustrated. Each ground strip includes a set of press-fit contact tails. The contact tails extend into through holes in the printed circuit board and make electrical contact with a ground plane in printed circuit board 2. In the illustration of FIG. 4, the opposite strip bar side of each ground strip is seen extending upward toward the viewer from the plane of the page. The contact tails (not seen) of the ground strip extend into the plane of the page. Motherboard connector 4 is made by inserting the signal pins and ground strips into accommodating holes and slots in insulative housing 7. See U.S. Pat. No. 6,872,085 for additional details.
To facilitate the design of transmission lines having constant characteristic impedances, signal conductors and dielectrics and ground planes are realized that have preset physical forms and orientations with respect to one another. One such set of forms and orientations is illustrated in cross-section in FIG. 5 (Prior Art). The signal conductors 16 and 17 within the dielectric 18 of a printed circuit board are disposed between two ground planes 19 and 20. In the diagram, two coupled stripline conductors 16 and 17 extend parallel to one another into the plane of the page.
FIG. 6 (Prior Art) illustrates another form and orientation called microstrip. In this form and orientation, there is one ground plane 20 disposed on one side of a pair of signal conductors 21 and 22, and the signal conductors are embedded in dielectric material 23 of the printed circuit board.
The stripline and microstrip forms of signal conductors, dielectric and ground planes are employed in the design of male motherboard connector 4 of FIG. 4. Note the similarity in appearance between the ground strips and signal conductor pins of the connector of FIG. 4 and the ground planes and signal conductors of the printed circuit boards of FIGS. 5 and 6.
FIG. 7 (Prior Art) is a simplified cross-sectional diagram that shows the female daughterboard connector 5 aligned with respect to the male motherboard connector 4. Female daughterboard connector 5 includes an insulative housing 24 and a set of signal conductors. Signal conductor 25 is referred to as an example. Signal conductor 25 terminates at one end in a press-fit contact tail 26 that extends into an associated through hole in the printed circuit board of daughterboard 3. Signal conductor 25 terminates at the other end in a pair of contact beams 27. When the two connectors 4 and 5 of the assembly are mated, pin 8 of male connector 4 extends through a hole 29 in insulative housing 24 and slidingly engages contact beams 27 so as to make electrical contact with signal conductor 25. Once mated, an electrical signal can pass from a conductor (not shown) within motherboard 2, through the contact tail 10 of pin 8 of motherboard connector 4, through pin 8 and to contact beams 27 of signal conductor 25, through signal conductor 25 in daughterboard connector 5, through the contact tail 26 and into a signal conductor (not shown) within daughterboard printed circuit board 3.
Daughterboard connector 5, in one embodiment, is made of multiple “wafers”. See U.S. Pat. No. 6,872,085 for further details. The signal conductors of one such wafer are illustrated in FIG. 7.
FIG. 8 (Prior Art) is an exploded view of one wafer. The wafer includes a shield plate of metal 31, insulative housing 24 and signal conductors 33. Signal conductor 25 is one of signal conductors 33. The metal signal conductors can be made by stamping them out of a metal plate. The metal plate is typically a thick, approximately 0.2 millimeter thick, stiff sheet of copper or copper alloy. The stamped metal signal conductors 33 are pressed into accommodating slots in insulative housing 24. Similarly, shield plate 31 can be stamped out of a sheet of metal and can be pressed into an accommodating recess in insulative housing 24. Many such wafers are stacked together so that the holes (for example, hole 29) in the insulative housings of the wafers align to form a two-dimensional matrix of holes. The stack of wafers is held together in place by a conductive stiffener clip (not shown). See U.S. Pat. No. 6,872,085 for further details.
Although this type of connector assembly works well in many environments, there exist problems in certain applications due to mismatches between connectors when motherboard and daughterboard connectors are brought together when printed circuit boards of electronic equipment are to be connected to one another. FIG. 9 (Prior Art) illustrates one such problem. Due to shortcomings in some printed circuit board fabrication techniques, a separation 28 between two daughterboard connectors 5 and 34 may vary in a range of plus or minus 0.1 millimeters. Similarly, a separation 30 between two motherboard connectors 4 and 35 may also vary in a range of plus or minus 0.1 millimeters. When daughterboard 3 and motherboard 2 are brought together, there can be a significant mismatch between connectors of each connector assembly. When the connectors are mated, the misalignment gives rise to mechanical stress between the connectors and the printed circuit boards to which they are attached. This mechanical stress must be absorbed satisfactorily without breaking the connectors or structures by which the connectors are attached to the printed circuit boards.
FIG. 10 (Prior Art) is a cross-sectional diagram illustrating such stress. The pin that extends downward and terminates in contact tail 36 is strong and absorbs stress due to connector 37 being pushed in the direction of arrow 38 with respect to printed circuit board 39 being pushed in direction of arrow 40. As signal frequencies increase, however, the length of such a contact tail and the associated plated through hole and the irregular shape and discontinuous electrical characteristics of the contact tail and plated through hole cause electrical reflections, cross-talk and/or electromagnetic radiation. Although strong and reliable, the structure of FIG. 10 is undesirable due to its electrical characteristics.
FIG. 11 (Prior Art) is a simplified cross-sectional diagram of an alternative structure wherein connector 37 is surface mounted to printed circuit board 39. A solder ball or surface mount connector pin 41 on connector 37 is soldered to a solder pad 42 on printed circuit board 39 by a solder joint 43. This structure does not have the irregularly shaped contact tail of FIG. 10, but the structure does have a somewhat long and conductive plated through hole 44. Plated through hole 44 may act as an antenna is an undesirable way. To help avoid this problem, a backdrilling step may be employed to remove much of the plated through hole 44. The dashed line 45 in FIG. 12 (Prior Art) illustrates the hole that results after back drilling.
FIG. 13 (Prior Art) illustrates another structure wherein expensive back drilling step is not needed. In the structure of FIG. 13, stacked blind vias or conductive plugs 46, 47 and 48 are built into printed circuit board 39 to connect surface mounted connector 37 to electrical conductor 49 within printed circuit board 39. Although it may be desired to be able to have the improved electrical properties of the surface mount structures of FIGS. 11–13 in a connector assembly design, stress due to the misalignment of connectors may cause solder joints between connector 37 and printed circuit board 39 to fail. The stress may lift the solder pad 42 off printed circuit board 39. It is therefore difficult or impossible to employ the surface mount techniques in high speed connector assemblies involving many signal pairs where there may be multiple connectors on each printed circuit board. An improvement upon the connector assembly structure of U.S. Pat. No. 6,872,085 is desired.